1. Field of the Invention
This invention relates to the general field of optical communications and, in particular, to a free-space, hitlessly switchable, optical interleaver.
2. Description of the Prior Art
In optical communications, one fiber can carry many communication channels. Each channel has its own carrier frequency. The light of different frequencies is merged into the fiber through a device called multiplexer (“mux”) in the art and is later separated into different ports through a device called de-multiplexer (“de-mux”). The mux and de-mux devices typically utilize technologies such as thin-film filters (TFF) and array wave-guide gratings (AWG).
Thus, in dense wavelength division multiplexing (DWDM) optical communication, various frequencies (1/λ) of laser light are used as carrier signals (channels) and are coupled into the same optical fiber, which acts as a waveguide. Data signals are superimposed over the carrier signals and are transported in the waveguide. Since the total usable wavelength range is limited (about a few tens of nanometers), as channel spacing is decreased, more channels can fit into the same optical fiber and greater communication capacity is achieved. Therefore, the ability to operate at ever reduced channel spacing is an important objective in the art.
However, channel spacing is limited by the capability of the multiplexer and the de-multiplexer to combine and separate channels without signal overlap. Currently, the standard for channel spacing is 100 GHz (0.8 nm) and manufacturing costs increase dramatically to implement a channel spacing smaller than 100 GHz.
Various methods are known to multiplex and de-multiplex signals with different carrier frequencies (wavelengths). When the total number of channels is less than about 20, the technology based on thin-film filtering is preferred because of its wide bandwidth, its good thermal stability, and the facility with which channels may be added to the system. However, since the channels are de-multiplexed by cascading filters in series, the insertion losses are not uniform among the various channels. In addition, when the channel spacing is about 50 GHz or smaller, narrow-band filters based on thin-film technology add too much chromatic dispersion for some applications.
Therefore, when the number of channels is high (e.g., more than about 40), it has been preferable in the art to use optical devices that provide a more uniform loss throughout the channels and exhibit a smaller chromatic dispersion than thin-film technology. For example, devices based on array waveguide grating (AWG) and diffraction grating provide these advantages. However, such devices tend to produce a narrower bandwidth than thin-film technology, which severely limits their application. In turn, a cost-effective method for increasing the bandwidth of multiplexing and de-multiplexing devices with uniform insertion loss throughout the channels and minimal chromatic dispersion has been achieved through the use of optical interleavers.
With an interleaver, it is possible to use lower resolution filters to mux/de-mux channels with a channel spacing that is smaller than the filter's frequency resolution. For instance, to de-mux eighty channels with a channel spacing of 50 GHz, the interleaver first separates the light into an odd stream and an even stream. The odd stream contains odd-number channels and the even stream contains even-number channels. By doing that, the channel spacing in each stream becomes 100 GHz. Therefore, one can use 100 GHz filters to separate the channels in either stream. Otherwise, one would have to use 50 GHz filters, which are more expensive than 100-GHz ones, to de-mux all 80-channel optical signals.
A conventional free-space de-mux interleaver is a 3-port device. As shown schematically in FIG. 1 in a Michelson interferometer configuration, an optical de-mux interleaver 10 includes a 50/50 beamsplitter 12 combined with a mirror 14 and phase optics 16. A single incoming light beam A is incident on a common (input) port 18 and two output beams B,C exit from respective output ports 20,22. A portion of the incoming beam A is first reflected at point 24 of the beamsplitter, and then it is reflected by the mirror 14 and returned to the beamsplitter at point 26, where it is reflected again and transmitted on a 50/50 energy split. The beam returned to point 26 has a phase that is a linear function of its optical frequency. The other portion of the incoming beam A at point 24 of the beamsplitter is transmitted to and phase shifted by the phase optics 16; then it is returned to the beamsplitter at point 26, where itself is also reflected and transmitted on a 50/50 energy split. This beam returned to point 26 has phase that is a nonlinear function of its optical frequency. The phase difference between the linear phase produced by mirror 14 and the nonlinear phase produced by the phase optics 16 determines which optical frequencies (wavelengths) are in the passband and in the stopband at each of the output ports B and C.
The preferred phase optics 16 consists of a GT (Gires-Tournois) etalon, as illustrated in FIG. 2. The etalon 16 includes a cavity 30 defined by two opposing optical surfaces 32 and 34 on respective optical elements 38 separated by precisely sized spacers 36. The first surface 32 is transmissive and coated with a partially reflective (PR) coating (typically 2-20% reflective), while the second surface 34 is coated with a totally reflective coating. As those skilled in the art readily understand, the length of the spacers 36 and the PR coating are tailored to produce the desired nonlinear phase.
As a result of this arrangement, the beams transmitted and reflected at point 26 interfere constructively and destructively to produce output beams B and C, such that beam B includes all 100 GHz ITU-grid frequencies of the incoming beam A and beam C includes all 50 GHz off-grid channels. Therefore, the device of FIG. 1 functions as a de-mux interleaver with A as an input and B and C as outputs for even and odd channels, respectively. The transmission spectra of the two output ports are complementary to one another. (As used herein, complementary means that light from the common port 18 goes to either output port 20 or output port 22 as a result of energy conservation.) FIG. 3 illustrates the components of an actual free-space Michelson interferometer interleaver 40, wherein the mirror surface is produced by combining a reflective surface 14 with spacers 42 and a transmissive element 44 in optical contact with a beamsplitter cube 46. A GT etalon 16 is similarly placed in optical contact with the cube 46 and the spacer lengths L1 and L2 are selected so as to produce the desired output beams as a result of interference. FIG. 4 illustrates the typical spectra of the beams B (even channels) and C (odd channels) generated at the two output ports when a PR coating of 14% reflection is used in the GT etalon.
This kind of interleaver is very good for applications where a continuous de-mux mode of operation is required. However, some applications require a switchable mode of operation between de-mux and total-pass (i.e., all channels are transmitted to a single output) modes. This is an inexpensive way, for instance, to upgrade a communication system from 100 GHz to 50 GHz. In such cases, it is crucial to be able to switch between modes without loss of signals during the transition phase, an event referred to as a “hit” in the art. Such hits cannot be tolerated in most optical networks. Therefore, this invention is directed at providing a “hitless” switchable de-mux device.